xprtrdma: Clean up after adding regbuf management

rpcrdma_{de}register_internal() are used only in verbs.c now.

MAX_RPCRDMAHDR is no longer used and can be removed.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>